Spatial light modulators (SLMs) are commonly used for imaging applications, both for display and printing. In general, an SLM is an array of pixel elements, which are individually addressable, usually with electronic signals. Many SLMs are binary, having an addressing scheme that switches the pixels to either an "on" or an "off" state to form the image. Various modulation and integration schemes are used to provide greyscale images. For printing applications, the SLM is used to expose a photoreceptor drum and can be addressed so that its pixels selectively emit or reflect light to the drum in accordance with the desired image.
One type of SLM is a digital mirror device (DMD), sometimes referred to as a deformable mirror device. The DMD has an array of hundreds or thousands of tiny tilting mirrors, which are the pixels. To permit the mirrors to tilt, each is attached to one or more hinges mounted on support posts, and spaced by means of an air gap over underlying control circuitry. The control circuitry provides electrostatic forces, which cause each mirror to selectively tilt.
For printing applications, SLMs are typically used to generate long and narrow images, which expose a given number of rows on the photoreceptor drum. For example, a typical SLM might be an inch long with rows of 900 pixels across its length. With appropriate magnification, an SLM of this size is suitable for exposing a 3 inch long strip of the image at 300 dots per inch (dpi).
However, it is often desired to have an image that is longer across than 3 inches. Although this could be accomplished by either adding more pixels so that the SLM is longer or by increasing the magnification, neither of these alternatives is desirable. A better technique for increasing the image size is described in U.S. Pat. No. 5,105,299, entitled "Unfolded Optics for Multiple Row Deformable Mirror Device", assigned to Texas Instruments Incorporated. The patent describes using a DMD to simultaneously generate two images, one above the other. One image is the left half of the line to be printed, and one image is the right half of the line. An optical guide is used to project both images side-by-side in their correct alignment.